Method and process of contact to a heat softened solder ball array

ABSTRACT

A method for enhancing temporary solder ball connection comprises the application of thermal energy to the solder balls, heating them to a submelting “softening” temperature, whereby the compression force required to connect all balls in a BGA is achieved at much reduced force, avoiding damage to the package, insert, substrate and support apparatus. Several forms of heating apparatus, and temperature measuring apparatus are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/651,664,filed Aug. 29, 2003, pending, which is a continuation of applicationSer. No. 10/196,396, filed Jul. 15, 2002, now U.S. Pat. No. 6,614,003,issued Sep. 2, 2003, which is a continuation of application Ser. No.09/892,156, filed Jun. 26, 2001, now U.S. Pat. No. 6,420,681, issuedJul. 16, 2002, which is a continuation of application Ser. No.09/618,885, filed Jul. 18, 2000, now U.S. Pat. No. 6,329,637, issuedDec. 11, 2001, which is a continuation of application Ser. No.09/145,832, filed Sep. 2, 1998, now U.S. Pat. No. 6,121,576, issued Sep.19, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor chip packages. Moreparticularly, the present invention pertains to methods for electricalcontact of an array of solder balls with a noncompliant surface.

2. State of the Art

The testing of packaged semiconductor devices has always presentedproblems to device manufacturers. Various types of tests may beconducted at different stages of manufacture. In the current state ofthe art, “wafer sort” electrical tests may be conducted prior topackaging to determine nonworking dies. Following packaging, varioustests including environmental tests as well as parametric and functionalelectrical tests may be performed. A final test which is known as“burn-in” may optionally be conducted. The test includes temperaturecycling over an extended period of time. Essential to the testing ofindividual dies is reliable electrical connection of all die leads tothe test board, without incurring damage to the die or testingapparatus, and easy disassembly from the testing apparatus. While“permanent” wire connections are widely used, wirebonding is timeconsuming and expensive, and also makes the matching of device impedanceto the substrate impedance very difficult to achieve. Much effort isbeing spent on developing alternative methods to reduce the time andexpense of using wire bonds. The replacement of wire bonds with ballgrid array (BGA) connections is becoming more common. Temporaryconductive attachment of solder balls to e.g., a test board is less thansatisfactory.

Temporary connection of device circuits to a test apparatus is known topresent a variety of problems. The insert member into which asemiconductor die is placed for testing is typically noncompliant, i.e.,ceramic or silicon, for example.

The current method for joining a ball grid array (BGA) to anoncompliant, i.e., rigid surface such as a silicon micromachined pocketinterconnect or insert, is to apply, at ambient temperature, arelatively high compression force of about 22-30 grams-force per solderball. Theoretically, all balls of the array should be pressed intomechanical and electrical contact with the insert pocket. The use ofcompressive forces lower than the above results in a further increasedfrequency of unsatisfactory electrical connections.

The presence of such unconnected solder balls in a BGA attachment formedunder ambient conditions is believed to be due to a significantvariability in ball diameter and “height” which the industry has beenunable to eliminate. As a result, the applied force of about 22-30grams-force or even more per ball is, in practice, insufficient toensure the required contact of all balls of the array. Furthermore, theuse of compression forces in excess of about 30 grams-force tends todamage the underlying material of the die, insert, and/or substrate. Forexample, effective connection of a 48 ball BGA array using solder ballsof a nominal diameter may require in excess of about 1.5 kg-force. Suchpressures exerted on a die for connection to a ceramic insert may damagethe die and/or insert and/or substrate below the insert. The total forcerequired for connection of larger arrays will be even more. In addition,the use of larger balls not only increases the absolute variation inball diameter but the force required to sufficiently deform each ballfor establishing the required temporary electrical connection. Theproblem also exists with smaller solder balls such as comprise a fineball-grid-array (FBGA) of 0.0125 inches (0.325 mm) diameter balls, forexample. With the smaller diameter solder balls, variation in ballplacement location may have a greater effect than nonuniform balldiameters.

To date, the industry has continued to use relatively high compressiveforces and necessarily accepted the increased occurrence of electricalconnection failures of a BGA and/or damage to the die, insert orsubstrate.

Ball grid arrays are used in a variety of semiconductor devices.Illustrative of such prior art are U.S. Pat. No. 5,642,261 of Bond etal., U.S. Pat. No. 5,639,695 of Jones et al., U.S. Pat. No. 5,616,958 ofLaine et al., U.S. Pat. No. 5,239,447 of Cotues et al., U.S. Pat. No.5,373,189 of Massit et al., and U.S. Pat. No. 5,639,696 of Liang et al.

Semiconductor devices having dual sets of outer “leads,” e.g., twin BGAsurfaces or a combination of e.g., J-leads and solder bumps, are shownin U.S. Pat. No. 5,648,679 of Chillara et al., U.S. Pat. No. 5,677,566of King et al., and U.S. Pat. No. 5,668,405 of Yamashita.

Chip carriers of several configurations are described in U.S. Pat. No.4,371,912 of Guzik, U.S. Pat. No. 4,638,348 of Brown et al., andJapanese publication 60-194548 (1985).

Semiconductor devices joined in stacks are disclosed in U.S. Pat. No.4,868,712 of Woodman, U.S. Pat. No. 4,841,355 of Parks, U.S. Pat. No.5,313,096 of Eide, U.S. Pat. No. 5,311,401 of Gates, Jr. et al., U.S.Pat. No. 5,128,831 of Fox, III et al., U.S. Pat. No. 5,231,304 ofSolomon, and U.S. Pat. No. 4,956,694 of Eide.

U.S. Pat. No. 5,637,536 of Val discloses a chip stacking configurationwith solder ball connections.

U.S. Pat. No. 5,012,323 of Farnworth discloses a dual-die package havingwire interconnections.

U.S. Pat. No. 4,761,681 of Reid discloses a multi-chip device havingelevated (conductor covered mesa) interconnections.

Despite the advanced state of the art in lead interconnection, devicepackaging and testing, the temporary connection of semiconductor devicesto testing apparatus and burn-in boards remains an area which needsimprovement.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to methods for electrical contact of anarray of solder balls with a noncompliant surface, that is, themechanical and electrical contact of a ball grid array (BGA) to arelatively noncompliant contact set such as a silicon micromachinedpocket interconnect (i.e., “insert”) for a test pad or burn-in board(BIB).

The present invention further provides a reliable BGA connection methodand apparatus whereby the required pressure is much reduced to eliminateor significantly reduce compression-caused damage to the die, insertand/or substrate.

The present invention comprises methods and apparatus for softeningsolder bumps or balls so that all of the bumps/balls in an array readilyconform to a matching array of conductive contact pockets or pads inanother body. The array of solder bumps/balls is heated to a softeningtemperature lower than the melting point of the solder and quicklyplaced in slightly compressed engagement with the contact pockets orpads of a substrate. As compared to joining the arrays at ambienttemperature, all bumps/balls of the BGA are reliably connected, and theconnection is achieved at a much reduced pressure, avoiding damage tothe die and/or substrate. In addition, much less stress is placed on theapparatus holding the packaged die, the insert and test board.

The softening temperature to which the solder is heated is below themelting temperature of the solder alloy.

A variety of heating apparatus and methods is disclosed, includingdirect heating of the bumps/balls, heating of the entire assembly,heating of a chuck holding the IC, heating of a chuck holding theinsert, direct heating of the insert or substrate, etc. A temperaturesensing circuit may also be incorporated into the insert, substrate, orsubstrate retaining socket for the purpose of measuring and controllingthe temperature to which the bumps/balls are heated.

While electrical contact is readily maintained during electrical testsor burn-in by maintaining a small compressive force, ball contact iseasily removed by discontinuing the compressive force and lifting theBGA from the insert or substrate to which it was electrically connected.

The invention is applicable to a wide variety of solder compositions,solder bump designs and ball diameters.

Other features of the invention will become clear from study of thefollowing description and related figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is illustrated in the following figures, wherein theelements are not necessarily shown to scale:

FIG. 1 is a perspective view of an insert assembly for the electricaltesting of a typical flip-chip semiconductor package with BGA, whereinheat enhancement of the BGA connection in accordance with the inventionis shown;

FIG. 1A is an edge view of a ball grid array on a semiconductor chip;

FIG. 2 is a perspective view of an insert assembly for the electricaltesting of a typical flip-chip semiconductor package with BGA, showingthe package in compressive engagement with the insert assembly forheating enhancement of the BGA connection in accordance with theinvention;

FIG. 3 is a plan view of a substrate member of the invention;

FIG. 4 is a bottom view of a substrate member of the invention;

FIG. 5 is a cross-sectional view of a portion of a heating assembly ofthe invention; and

FIG. 6 is a cross-sectional view of various solder ball contact sites towhich the invention may be applied.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to method and apparatus embodiments forthe uniform temporary electrical connection of solder bumps, e.g.,solder balls, of a semiconductor device to another body. Rapid thermalsoftening of the solder bumps may be achieved by a variety of specificmethods and apparatus, as described herein. The methods are particularlyuseful for attachment of solder bumps to the surface of a noncompliantbody such as formed of silicon, ceramic, etc.

As shown in drawing FIG. 1, a semiconductor package 10 is exemplified bya flip-chip package (FCP) with a ball grid array (BGA) 30 of a pluralityof solder bumps or balls 12 on one surface 14 of the semiconductorpackage 10.

A test apparatus for evaluating circuit performance of the semiconductorpackage 10 is shown as including an insert 16 and a substrate member 18.The insert 16 is noncompliant and is typically formed of ceramic orsilicon with a pattern of electrical contact sites 20 micromachined onits upper surface 22. The contact sites 20 may comprise simple planarpads, or contact pockets of any configuration, as explained infra. Thecontact sites 20 are connected by conductive traces, not visible, tobond pads 24, the latter being connected by wire bonds 26 to conductivetraces 28 on the substrate member 18. The wire bonds 26 and conductivetraces 28 on the insert 16 and substrate member 18 may be encapsulatedin resin for protection. Other means for connecting the contact sites 20to a controller for conducting a test, burn-in, etc., may be used, asknown in the art.

The substrate member 18 and attached insert 16 are typically insertedinto a socket on a test fixture or a burn-in board (BIB), neither shownin drawing FIG. 1.

In accordance with the invention, the ball grid array 30 of solderbumps/balls 12 is heated and compressed under a slight pressure into thecontact sites 20, shown here as indentations or pockets. The solderbumps/balls 12 are heated to a submelting softening temperature T_(s)and are uniformly contactable to the contact sites 20 by an increaseddeformation under the slight compression force.

In one simple embodiment, an external heater 40 emitting infraredradiation or heated air 42 is positioned to heat the semiconductorpackage 10 including the solder bumps/balls 12 to the desired softeningtemperature, and the BGA 30 is quickly inserted and compressed by force38 into engagement with the contact sites 20 at a relatively lowpressure such as about 2-10 g-force per solder bump/ball 12. Referringto drawing FIG. 1A, of course, the required force per solder bump/ball12 will vary, depending upon the softening characteristics of theparticular solder composition used, the temperature to which the solderbumps/balls 12 are heated, the nominal ball diameter 32, the maximumvariation in ball diameter 32 and the variation in drop distance 34between ball centers 34A and the surface 14 of semiconductor package 10.Typically, the required compression force 38 at the softeningtemperature T_(s) to achieve complete ball connection is about 8-25percent of the force at ambient temperature.

Instead of directly heating the semiconductor package 10 to soften thesolder bumps/balls 12, heat may be applied to the insert 16 or substratemember 18 before connecting the BGA 30 to the contact sites 20. Also,the semiconductor package 10 may be indirectly heated by applyingthermal energy to a chuck, not shown, which holds the package.

As shown in drawing FIG. 2, a semiconductor package 10 with an array ofsolder bumps/balls 12 is placed on an insert 16, and placed under acompression force 38. Thermal energy is applied either to the back side36 (as shown in FIG. 1) of the semiconductor package 10, to the insert16, to the substrate member 18 (as shown in FIGS. 4 and 5), to acompression member, not shown, compressing the back side of thesemiconductor package 10 with compression force 38, or to a socket, notshown, which surrounds the substrate.

Alternatively, the assembly of semiconductor package 10, insert 16 andsubstrate member 18, together with compression and support apparatus,may be placed in a temperature controlled oven and rapidly heated to thedesired softening temperature T_(s).

Thus, the solder bumps/balls 12 may be heated by conduction, convectionor radiation, or any combination thereof. For example, an externalheater 40 (FIG. 1) may heat the semiconductor package 10, insert 16,substrate member 18, or a socket 66 into which the substrate member 18fits by radiation or heated air 42.

The solder bumps/balls 12 may be of any diameter 32, including those ofa fine ball grid array (FBGA), where the balls have a pitch of less thanone (1) mm.

The solder bumps/balls 12 may be formed of various solder compositions,including tin-lead solders having a lead content of about 30 to 98percent. Solder compositions having the higher lead concentrations oftenhave a higher melting point.

A softening temperature Ts of about 130° C. to about 180° C. has beenfound useful for reducing the compression force 38 to a relatively lowvalue and simultaneously ensuring electrical contact of all solderbumps/balls 12.

As shown in drawing FIG. 3, resistive heating elements 44 may be appliedto the top surface 48 of the substrate member 18, preferably under theinsert 16 and substantially beneath the semiconductor package 10. Theheating elements 44 are shown as having heater power leads 54, 56 forproviding sufficient power to quickly heat the insert 16 including theelectrical contact sites 20, not shown, and the solder bumps/balls 12,not shown, which are in engagement with the contact sites 20.

All of the conductive traces on substrate member 18, includingconductive traces 28, heater power leads 54, 56, and heating elements 44may be formed simultaneously by screening a thick film of conductivematerial onto the substrate member. This method of forming conductivetraces on a surface is well known in the art.

A thermocouple junction 50 or other temperature detecting device may beinstalled in or on the insert 16 or substrate member 18 for obtainingtemperature feedback and controlling the bump/ball temperature to attaina maximum desired softening temperature T_(s). Thus, for example, asshown in drawing FIG. 3, a temperature sensor 50 (such as a thermocouplejunction) may be fixed on the top surface 48 of the substrate member 18or back side 52 (FIGS. 1 and 2) of the insert 16, and have thermocoupleleads 58 connected through otherwise unused conductive traces 28A, 28Bto measurement/control instrumentation, not shown. In use, a heatercontroller, not shown, determines the measured temperature and shuts off(or reduces) power to the heating elements 44 upon sensing apredetermined temperature. A recorder, not shown, may be used tocalibrate the measurements such that a desired softening temperature maybe precisely attained.

A short heating time is preferred, extending only several seconds orless. Most preferably, the heating time is less than one second. Thus,the heater power leads 54, 56 to the heating elements 44 must besufficiently large to carry the necessary electrical load. In general,installation of the heating elements 44 on the insert 16 will requireseparate heater power leads 54, 56. Normally, wire bonds 26 (FIG. 1) areincapable of carrying the necessary load.

Another form of heating apparatus which may be used in the invention isillustrated in drawing FIGS. 4 and 5. The substrate member 18 has on itsback side (underside) 46, as shown in FIG. 2, a pattern of heatingelements 44 with junctions 62, 64. The junctions 62, 64 may be planarpads or conductively surfaced indentations in the back side 46.

As shown in drawing FIG. 5, a semiconductor package 10, insert 16, andsubstrate member 18 are positioned in a socket 66 on a test board 70.Test board 70 may be a board for an electrical test, for burn-in, orother purpose. The socket 66 is typically formed with walls 68 and base72, and many sockets 66 may be mounted on a single test board 70 toenable simultaneous testing or burn-in of many semiconductor packages10.

A pair of through-holes 74, 76 is formed in the test board 70 along axes84, 86, and the axes which pass through junctions 62, 64, respectively.Two metal spring-loaded compression pins 80, also known as “pogo pins,”are mounted in the test board 70 or in another substrate 90 underlyingthe test board 70. Substrate 90, having a plurality of pogo pins 80projecting therefrom, is known as a bed-of-nails (BON). The pogo pins 80have a base 78 and a spring-loaded pin 82 which is axially movablerelative to the base 78. The spring-loaded pins 82 are shown passingthrough-holes 74, 76 to electrically contact the junctions 62, 64 whenin compression, power leads 92, 94 from the two pogo pins 80 providingsufficient electric power to the heating elements 44 for rapidly heatingthe solder bumps/balls 12. Following testing, the spring-loaded pogopins 80 will push the substrate member 18 from the socket 66 with ashort stroke.

In drawing FIG. 6, several types of BGA contact sites 20 are shown asexamples illustrating the wide variety of solder bumps/balls 12 andcontact sites 20 combinations whose temporary connection is enhanced byuse of an elevated submelting softening temperature T_(s). Each solderbump/ball 12 attached to semiconductor package 10 is configured to be incompressive conductive contact with a contact site 20.

Contact site 20A comprises a flat pad or surface of the insert 16.

Contact site 20B is a spherical indentation in the insert 16.

Contact site 20C is a shallow spherical indentation.

Contact site 20D is a spherical indentation having a central axiallydirected projection 96 which punctures and enters the softened solderbump/ball 12. Preferably, the projection 96 is pyramidal in shape.

Contact site 20E is a spherical indentation having several, typicallyfour, peripheral projections 98 which contact and are forced into thecircumferential surface of the solder bump/ball 12.

The illustrated contact sites 20 to which the invention may be appliedare exemplary only and not exhaustive.

It is clear that a wide variety of apparatus may be used for heatingball-grid-array connections, of which those described herein arerepresentative.

The invention has been illustrated in application to the testing of aflip-chip device. However, the temporary BGA connection of any device,including other chip scale packages (CSP), is enhanced by this processand apparatus.

It is apparent to those skilled in the art that various changes andmodifications, including variations in heating procedures andstructures, may be made to the BGA connection method and apparatus ofthe invention as described herein without departing from the spirit andscope of the invention as defined in the following claims.

1. A method for forming a portion of a solder bump of an array of solderbumps of a device to a contact site of a plurality of contact sites of amember, comprising: providing the solder bump of the array of solderbumps at a temperature Ts below a melting temperature of the solderbump; and moving the solder bump of the array of solder bumps of thedevice using a pressure less than substantially 22 grams-force forcontacting the contact site.
 2. The method of claim 1, wherein themelting temperature of the solder bump of the array of solder bumps isT° C. higher than an ambient temperature To, and wherein the softeningtemperature Ts is in the range of about 0.5 T to 0.95 T above theambient temperature To.
 3. The method of claim 1, wherein the solderbump of the array of solder bumps contacts the contact site of theplurality of conductive contact sites at a pressure not substantiallyexceeding about 10 grams-force.
 4. The method of claim 1, wherein thesolder bump of the array of solder bumps contacts the plurality ofconductive contact sites at a pressure in the range of about 2 to 10grams-force.
 5. The method of claim 1, wherein the semiconductor devicehaving the array of solder bumps is heated by one of hot air convectionand infrared radiation.
 6. The method of claim 1, wherein the memberhaving the plurality of conductive contact sites is heated by one of hotair convection, conduction from a heated object, and infrared radiation.7. The method of claim 1, wherein the semiconductor device and themember are placed in a temperature-controlled oven for heating to thesoftening temperature Ts.
 8. The method of claim 1, wherein thesemiconductor device is held in a chuck, the chuck being heated.
 9. Themethod of claim 1, wherein the member is held in a chuck, the chuckbeing heated.
 10. The method of claim 1, wherein the member having theplurality of conductive contact sites is heated by electrical resistancewires.
 11. The method of claim 1, wherein the member and a substrate aremounted on a mounting board having an integral heater, the integralheater controlled to heat the member to the softening temperature Ts.12. The method of claim 1, wherein the array of solder bumps comprisesSn—Pb solder having a lead content in the range of about 40 to about 98percent, and the softening temperature Ts comprises a range of about 140to 180° C.
 13. The method of claim 1, wherein heating comprisespredetermining a heating time X to heat the solder bump of the array ofsolder bumps to the softening temperature Ts, and heating for the timeX.
 14. The method of claim 1, wherein heating comprises initiating theheating, measuring a temperature of one of the member and thesemiconductor device, and stopping the heating to limit a temperature ofthe solder bump of the array of solder bumps to no more than thesoftening temperature Ts.
 15. An apparatus for connecting a solder ballto a contact site comprising: a first member having a solder ballthereon; a second member having a contact site; apparatus moving thefirst member against the second member for contact of the solder ball tothe contact site, the first member contacting the second member at apressure less than substantially 22 grams-force for the solder ball; andapparatus for increasing the temperature of the solder ball and thecontact site to a solder-softening temperature Ts.
 16. The apparatus ofclaim 15, wherein the contact site comprises one of a substantially flatsurface, a recess for receiving a portion of a solder ball, and a recesshaving at least one projection therein for deforming a solder ballinserted therein.
 17. Testing apparatus for a semiconductor packagehaving at least one solder ball on a surface thereof, the apparatuscomprising: an insert formed of generally noncompliant material having afirst surface including at least one contact site for contacting the atleast one solder ball and having a second surface; a substrate having afirst surface, having a second surface, the second surface of the insertsecured to the first surface of the substrate, and having at least onelead on the substrate for connecting to at least one lead in a socket;at least one electrical lead connecting the at least one contact site ofthe insert with the at least one lead of the substrate; a test boardhaving the socket with at least one contact lead connected to a testingcircuit, the substrate and the insert for insertion into the socket forcontact of the at least one lead of the substrate with the at least onecontact lead of the socket; and apparatus associated with at least oneof the substrate, the insert, and the socket to increase the temperaturethereof to a predetermined level.
 18. The apparatus of claim 17, furthercomprising temperature-sensing apparatus attached to one of thesubstrate, the insert, and the semiconductor package.
 19. The apparatusof claim 18, further comprising a temperature controller for controllingthe apparatus to increase the temperature of the substrate, the insert,and the socket.